The present invention relates in general to phased array ultrasound imaging, and more specifically to a phased array ultrasound probe with increased dynamic range.
Ultrasound imaging systems transmit ultrasonic waves into a volume to be imaged and receive ultrasonic waves therefrom. The reflected ultrasonic waves are converted to electrical signals which are then processed for use in controlling a video display. In an electronically steered sector scanner, ultrasonic waves are transmitted and received from multiple transducer elements, separately excited and arranged in an array on a probe. Types of electronically steered probes include linear arrays, two-dimensional arrays, curved phased arrays, and annular phased arrays, which will be collectively referred to as phased arrays.
Phased array scanners form images using electronic focusing to direct ultrasonic energy along a line (i.e., vector angle) and to receive echoes from the line. Focusing is achieved by selectively delaying signals during transmission and during reception by respective transducer elements in the phased array. Thus, signals along the vector angle exhibit constructive interference and signals from other locations exhibit destructive interference.
The ultrasound beam is steered along various vector angles by selectively delaying the signals according to the differences in path length between each transducer element and the various points on the beam line. Each transducer is typically connected to a respective controlled delay element. On reception, the output signals from the delay elements are coherently summed to provide an echo signal.
The time delay introduced by each delay element is determined by a controller, typically a computer or microprocessor. Many different schemes have been used to implement the needed signal delays. U.S. Pat. No. 4,285,011, issued to Sato, provides an example wherein analog switches connected to the taps of many delay lines are controlled by a CPU.
Due to physical manufacturing limitations, the transducer elements of an array have their own inherent time delays that vary from element to element. These variable delays modify the relative physical delay applied to a channel of the array such that the actual delay varies from the theoretical delay. Therefore, an image reconstructed from the delayed signals exhibits reduced dynamic range.
Because of the nature of transducer materials, it is not practical to isolate the cause of delay errors to any particular electrical or mechanical parameters. Thus, it has not been possible to eliminate delay errors by improving manufacturing techniques.
Accordingly, it is a principal object of the invention to improve the dynamic range of phased array ultrasound systems.
It is another object of the invention to provide a method and apparatus for removing signal delay errors caused by the transducer characteristics.